1. Field of the Invention
The present invention relates to a discharge lamp, and relates particularly to a discharge lamp wherein thorium (Th) is used as the emitter in the cathode.
2. Description of Related Art
Traditionally, high-pressure mercury lamps are used for the light source in exposure machines for liquid crystals or semiconductors, while xenon lamps are used in the light source for projectors. It is necessary for these discharge lamps that the arc is stable during the lighting (arc stability) and that a constant irradiance can be maintained for a long time (durability). To meet these demands, a material with excellent ability to ignite the arc and excellent wear resistance becomes necessary for the electrodes, and in particular so-called thoriated tungsten (ThO2—W) for which thorium oxide (ThO2) has been doped into tungsten (W) has been used for the material of the cathode (JP-A-42-27213 (1967)).
But in recent years restrictions for the use of radioactive substances such as thorium arc are to be observed with regard to the environmental load, whereas the arc stability and the durability rightly have been necessary for discharge lamps.